1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method and, for example, to an image processing apparatus and image processing method for outputting a pulse-width modulated signal to an image forming device using an electrophotographic method.
2. Description of the Related Art
An image forming device such as a copying machine or printer using an electrophotographic method charges a photosensitive drum, and causes a light beam to scan and expose the charged photosensitive drum, thereby forming an electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is developed by a color material (toner) so as to form a toner image. The toner image is transferred and fixed to a printing paper sheet, thereby forming a visible image on the printing paper sheet.
Such an image forming device uses various motors such as a motor for rotating the photosensitive drum. Hence, the output image degrades because of the influence of a fluctuation in the motor rotation speed, the eccentricity of a rotating shaft or gear, pitch error, and the like. For example, a vibration of the photosensitive drum or a fluctuation in the rotation speed during latent image formation makes the pitch of scanning lines fluctuate in a direction (the rotation direction of the photosensitive drum; to be referred to as a sub-scanning direction hereinafter) perpendicular to the laser beam scanning direction (to be referred to as a main scanning direction hereinafter). This fluctuation causes a density fluctuation called banding in an output image, resulting in a decrease in the output image quality.
U.S. Pat. No. 5,134,495 (patent reference 1) discloses one of the solutions to this problem. This method allows distributing of the density value of the dot of a pixel of interest to surrounding pixels in the sub-scanning direction, and thus shifting the position of each dot of the toner image in the sub-scanning direction.
The technique of patent reference 1 will be explained with reference to FIGS. 1A to 1E. FIG. 1A shows a state in which scanning lines K−1, K, and K+1 (K is an integer) are arranged in the sub-scanning direction, and a dot A is formed on the scanning line K−1. To help understanding the technique of patent reference 1, an example will be described in which, for example, the position of the dot A is shifted in the sub-scanning direction by a ½ pixel so as to correct pitch fluctuation.
FIG. 1B is a view schematically showing a pulse-width modulated (PWM) signal Sa corresponding to the dot A shown in FIG. 1A. Note that the pixel value of the dot A corresponds to 100% density (maximum value). In the technique of patent reference 1, to correct pitch fluctuation, each pixel value of the input image is distributed to pixels (to be referred to as adjacent pixels hereinafter) adjacent in a direction (to be referred to as a forward direction hereinafter) reverse to the rotation direction of the photosensitive drum in accordance with correction coefficients set in a table stored in the memory. For example, the pixel value of the dot A is distributed to pixels on the scanning lines K−1 and K in accordance with correction coefficients to generate PWM signals Sa1 and Sa2 shown in FIG. 1C. In the example of FIG. 1C, both the PWM signals Sa1 and Sa2 correspond to 50% density. Each of the PWM signals Sa1 and Sa2 is a signal pulse-width modulated to, for example, grow the pixel from the center of the pixel position.
FIG. 1D is a view showing laser irradiation after the pixel value distribution. The widths of rectangular regions La1 and La2 indicate the ranges of laser irradiations by the PWM signals Sa1 and Sa2. A region Ra represents the region of a latent image to be formed by laser irradiation according to the PWM signals Sa1 and Sat. A position D1 indicates the beginning end of one pixel; D2, the starting position of laser irradiations La1 and La2; D3, the end position of laser irradiations La1 and La2; and D4, the terminating end of one pixel. A latent image formed by the laser irradiation La1 and a latent image formed by the laser irradiation La2 are composited to form a latent image corresponding to the region Ra. When the latent image is developed, and a toner image is transferred and fixed, a dot A′ shown in FIG. 1E is formed. The dot A′ is shifted from the dot A by a ½ pixel in the forward direction (toward the scanning line K+1).
The technique of patent reference 1 can thus shift each dot position of a toner image in the sub-scanning direction, thereby correcting banding caused by pitch fluctuation. According to the technique of patent reference 1, a line formed from single dots or 1-dot width and running in the main scanning direction has high image quality after correcting pitch fluctuation. However, the width of a line of a 2-dot width or more extends after pitch fluctuation correction, resulting in poor image quality.
Image quality degradation that occurs in the technique of patent reference 1 will be described with reference to FIGS. 2A to 2E. FIG. 2A shows a state in which when banding has occurred, a line of a 2-dot width is then formed by two dots adjacent in the vertical direction. The dots A and B form a dot C that forms a line of a 2-dot width. Assume that the position of the line is shifted by a ½ pixel in the forward direction to correct pitch fluctuation. Both the dots A and B are assumed to have pixel values corresponding to 100% density (maximum value).
FIG. 2B shows PWM signals Sa1, Sab, and Sb2 generated by distributing the pixel values of the dots A and B to adjacent pixels in accordance with correction coefficients. The pixel value of the dot A is equally distributed to the pixels on the scanning lines K−1 and K. The pixel value of the dot B is equally distributed to the pixels on the scanning lines K and K+1. With this processing, the PWM signals Sa1 and Sb2 have values corresponding to 50% density, and the PWM signal Sab has a value corresponding to 100% density. Each of the PWM signals Sa1, Sab, and Sb2 is a signal pulse-width modulated to, for example, grow the pixel from the center of the pixel position.
FIG. 2C is a view showing laser irradiation after the pixel value distribution. The widths of the rectangular regions La1, Lab, and Lb2 indicate the ranges of laser irradiations by the PWM signals Sa1, Sab, and Sb2. The region Ra represents the region of a latent image to be formed by laser irradiation according to the PWM signal Sa1. A region Rab represents the region of a latent image to be formed by laser irradiation according to the PWM signal Sab. A region Rb represents the region of a latent image to be formed by laser irradiation according to the PWM signal Sb2. Note that the positions D1 to D4 are the same as in FIG. 1D.
When laser irradiation is performed as shown in FIG. 2C, a composite of the latent images formed by the laser irradiations La1, Lab, and Lb2, that is, a latent image corresponding to the regions Ra, Rab, and Rb shown in FIG. 2C is formed. At this time, not a latent image corresponding to 100% density but a latent image corresponding to 150% density is formed in the overlap region of the regions Ra and Rab. Similarly, a latent image corresponding to 150% density is formed in the overlap region of the regions Rab and Rb as well. That is, the toner image has excess toner in the overlap regions.
FIG. 2D is a view schematically showing the toner image formed on the photosensitive drum. The toner image includes a toner image Ta formed by developing the latent image in the region Ra, a toner image Tc formed by developing the latent image in the non-overlap region of the region Rab, and a toner image Tb formed by developing the latent image in the region Rb. Since excess toner exists in the above-described overlap region, dots of toner spread in the sub-scanning direction are formed by the toner images Ta and Tb, as compared to an image without the overlap region. Note that since a line is formed by the toner images Ta and Tb arranged in the main scanning direction, the toner images spread not in the main scanning direction with a high toner density, but in the sub-scanning direction with a low toner density, although not illustrated. As a result, a dot C′, thicker (for example, 3-dot width) than the original dot C of the 2-dot width is formed, as shown in FIG. 2E, resulting in a fattened line. According to the patent reference 1, the sharpness of the line is decreased.